Conventionally, a power transmission device for an automobile has hitherto been proposed, which is provided with a V belt variable transmission comprising a driving pulley, a driven pulley and an endless V belt running across the driving and driven pulleys, so that each pulley changes its sheave diameter to change a transmission gear ratio in a stepless manner.
The belt variable transmission constructed as above-mentioned is inferior in power transmission efficiency as compared to a usual gear transmission system.
In order to solve the above problem, for example, as disclosed in the Japanese Patent Laid-Open Gazette No. Sho 58-28,046, a power transmission device has been proposed which is provided between an input shaft connected to a drive source and an output shaft with a belt stepless variable transmission and a gear variable transmission disposed in parallel thereto so that the power is transmitted through the belt stepless variable transmission at the low stage speed change and through the gear transmission at the high stage speed change, thereby improving the power transmission efficiency at the high stage speed change for most running time so as to lengthen the life span of the belt.
The conventional gear transmission comprises a direct coupling driving gear provided in a fixed manner with respect to the input shaft, an idle gear engageable with the direct coupling driving gear, and a driven gear sleeved onto the output shaft and engageable with the idle gear. When the power is transmitted from the drive source to a differential gear through the gear transmission(hereinafter referred to as the direct coupling) the power from the input shaft is transmitted to the differential gear through the direct coupling driving gear, the idle gear, the driven gear, a main driving gear provided at the output shaft, and a counter driving gear.
Accordingly, power transmission by the direct coupling gear is carried out at four stages so that a power loss increases during the power transmission from the input shaft to the differential gear and the number of shafts and gears increases, thereby creating a problem in that the power transmission device is complicated in construction and of large size.
Meanwhile, the conventional power transmission device is provided on the output shaft with a forward gear and a reverse gear which are selectively coupled with the output shaft by a switching mechanism, the forward gear engaging with a forward only counter gear at a counter shaft disposed in parallel to the output shaft, the reverse gear engaging with a reverse only counter gear on the counter shaft through an idle gear on an idle shaft disposed in parallel to the output shaft.
Thus, on the counter shaft are provided the forward only counter gear and reverse only counter gear each of larger diameter, whereby these gears occupy a large space for arrangement and the entire device also is of large size.
Moreover, the number of teeth of the idle gear is irrelevant to a reduction ratio, so that the reduction ratio during the rearward running depends only on the number of teeth of the reverse gear and reverse only counter gear, thereby creating a problem in that a degree of freedom in design for setting the reduction ratio is restricted to that extent.
Also, the belt variable transmission will creat variation in the speed change ratio at the top speed due to variation in length of the belt or in axial length of a belt contact surface at each pulley, wear in the side surface of the belt with the lapse of time, or extension of the belt when subjected to a working load.
Hence, there are some cases where a speed change ratio during the direct coupling drive is lower than the speed change ratio at the top speed during the belt drive. In other words, the driving speed during the direct coupling drive is higher than the top speed during the belt drive. As a result, a problem is created in that, when the direct coupling drive is switched to the belt drive, a shock occurs.